1. Field of the Invention
The present invention relates to a shoulder anchor height adjusting apparatus for adjusting the height of a shoulder anchor by moving the shoulder anchor through which a longitudinal intermediate portion of a webbing is inserted, at an upper portion of a side wall of a vehicle in a vertical direction of the vehicle.
2. Description of the Related Art
In a three point seat belt for securing a passenger, one end of a webbing is wound up onto a webbing retractor which is mounted at a lower portion of a side wall of a vehicle. Further, the other end of the webbing is anchored by an anchor plate secured to the lower portion of the side wall of the vehicle through a shoulder anchor which is positioned at an upper portion of the side wall of the vehicle. A tongue plate is slidably fitted to the webbing between the shoulder anchor and the anchor plate. The tongue plate engages a buckle apparatus so that the webbing secures the passenger to the seat. The webbing between the shoulder anchor and the tongue plate forms a shoulder webbing, and the webbing between the tongue plate and the anchor plate forms a wrap webbing.
In order to adjust the shoulder webbing so as to conform to the body of the passenger, there is provided a shoulder anchor height adjusting apparatus for adjusting the height of the shoulder anchor.
In a conventional shoulder anchor height adjusting apparatus, as shown in FIG. 9, a long bolt 90 serving as a feed screw is disposed so as to have a vertical direction of the vehicle as its axial direction. Axial both ends of the long bolt 90 are rotatably supported at the upper portion of the side wall of the vehicle. A nut 92 serving as a screw member is screwed to the long bolt 90. A shoulder anchor 94 is mounted to the nut 92 by using an anchor bolt 98. The long bolt 90 is rotated by the driving force of a motor so that the nut 92 is moved with the shoulder anchor 94 in the vertical direction of the vehicle.
In the conventional shoulder anchor height adjusting apparatus, the inertia of the passenger causes forward movement when the vehicle rapidly decelerates. Accordingly, a fairly large load is exerted on the shoulder anchor 94 via the webbing 96. In this case, load in the vertical direction of the vehicle is absorbed by the feed screw. Consequently, the long bolt 90 and the nut 92 must be made of metal having high rigidity so as to ensure sufficient rigidity to resist the load in the vertical direction of the vehicle. However, if the long bolt 90 and the nut 92 are made of metal as described above, large frictional resistance is generated between the long bolt 90 and the nut 92 when adjusting the height of the shoulder anchor 94. Therefore, a large rotation torque is required for the long bolt 90. As a result, a large-sized motor becomes necessary, the weight of the shoulder anchor height adjusting apparatus increases, and production cost rises. In addition, operation noise becomes especially loud when a metal bolt 90 and metal nut 92 come in contact with each other.